gs-shell: Title of the window not updated with the current page title

The gtk_stack_get_visible_child() returns an old child when being called
inside stack_notify_visible_child_cb(), thus use the page variable instead,
which references the page being set as active.

Closes #1286
3 jobs for 1286-app-pages-no-longer-have-the-app-name-as-the-title in 11 minutes and 15 seconds (queued for 16 minutes and 34 seconds)